sunjbill saidyeah, i agree with the HIIT suggestion, get high quality cardio done in 20 minutes or less.
That does have its good points. It can fairly quickly lower the resting pulse rate, probably into the low 40s, and the resting pulse rate is a good indicator of cardiac fitness. There are multiple ways to do it, including intervals while running, bicycle riding, or swimming, during which one pushes oneself well into O2 debt. However, I think that there is also something to be said for endurance and that can be achieved only by prolonged exercise at an intensity somewhat below the maximum possible sustainable level.
Doing high intensity aerobic exercise can be dangerous unless one works up to it gradually. Even then, it can be dangerous if there are underlying cardiac problems, such as partially clogged arteries. Even so, if it is done properly, it should be safe and it will reduce the likelihood of cardiac problems.
Endurance is much more quickly achieved via HIIT and doesn't have the pathology of study death syndrome as does steady state cardio. This has been studied extensively. HIIT is vastly superior to steady state, low intensity, cardio, and improves cardiac endurance, as well as cardiac pump function in as few at 8 4 minute sessions.
You need to study this further to become informed of current day science.
Even in cardiac rehab, post surgical, two weeks out, they will take you to 85%, while on an EKG, because the results are so much better with HIIT.
If you have heart disease, this is a great way to find it. You may be an irregular ST segment, but, it's better for that to happen than to drop dead. Find heart disease before it finds you.
Your PASSIVE recovery should be 30 BPM in 90 seconds or better, if you don't have heart disease, FYI. (Mine is 35 BPM in 90, or even shorter.)
Note that you should walk, not stop, if you're having pain, or shortness of breath. Your legs help your heart by as much as 2/3 of your cardiac function (leg pump, it's called). Do NOT lay down unless you absolutely feel you are going to hit the floor. Walk, instead, to keep the blood moving, taking deep, long, breathes.
If you have severe pain in your shoulders, chest, traps, neck, and it doesn't calm down. CHEW a whole aspirin, and get to an EKG, where someone can look at your EKG. It could be nothing, or, you could be dying.